This invention relates generally to optical communication systems, and more particularly to a device and method for accurately determining PMD of optical signals transmitted over a fiberoptic network.
In a typical optical communication system, an optical signal in the form of a series of light pulses is emitted from an optical transmitter comprising a modulated laser diode. In the frequency domain, this signal comprises numerous frequency components spaced very closely about the nominal center frequency of the optical carrier, such as 193,000 GHz. As this type of modulated optical signal passes through an optical fiber, different frequency components of the optical signal travel at slightly different speeds due to an effect known as chromatic dispersion. In the course of an optical signal traveling through a very long fiber, such as 200 km, chromatic dispersion causes a single pulse of light to broaden in the time domain, and causes adjacent pulses to overlap one another, interfering with accurate reception. Fortunately, many techniques are known for compensating for chromatic dispersion.
Another form of dispersion is becoming a limiting factor in optical communication systems as progressively higher data rates are attempted. Polarization mode dispersion (PMD) arises due to birefringence in the optical fiber. This means that for two orthogonal directions of polarization, a given fiber can exhibit differing propagation speeds. A light pulse traveling through a fiber will probably, unless some control means are employed, have its energy partitioned into polarization components that travel at different speeds. As with chromatic dispersion, this speed difference causes pulse broadening and restricts the usable bandwidth of each optical carrier.
A modulated optical signal arriving at an optical receiver must be of sufficient quality to allow the receiver to clearly distinguish the on-and-off pattern of light pulses sent by the transmitter. Conventionally, a properly designed optical link can maintain a bit-error-rate (BER) of 10xe2x88x9213 or better. Noise, attenuation, and dispersion are a few of the impairments that can render an optical signal marginal or unusable at the receiver. Generally, when an optical channel degrades to a bit-error-rate of 10xe2x88x928, a communication system will automatically switch to an alternate optical channel in an attempt to improve the BER.
One common method of analyzing the quality of a modulated optical signal is a so-called xe2x80x9ceye diagram,xe2x80x9d shown in FIG. 1. The eye diagram consists of overlaying successive frames of time-domain traces of the optical signal, with each frame corresponding to one period of the nominal periodicity of the modulation. The vertical axis of an eye diagram represents instantaneous intensity (I) of the received signal, and the horizontal axis corresponds to time (T). Many successive traces of transmitted xe2x80x9conesxe2x80x9d and xe2x80x9czerosxe2x80x9d define a region or window within the middle of the display, defining the xe2x80x9ceye diagram.xe2x80x9d On the time axis, the window is bound on either side by the transitional leading and trailing edges of the pulses. The eye diagram in FIG. 1 shows an optical signal 50, 52 traveling through the optical fiber. A large clear area or xe2x80x9cwindowxe2x80x9d in the center, such as the one shown in FIG. 1, with a single peak and no encroachment from any side, represents a good signal in that the presence or absence of a pulse during each clock cycle is clearly distinguishable.
Noise added to a signal appears as xe2x80x9cfuzzinessxe2x80x9d of the lines defining the window. Sufficient noise can even obliterate the appearance of the window, representing a bad signal in that xe2x80x9conesxe2x80x9d and xe2x80x9czerosxe2x80x9d are no longer distinguishable. Impairments in the time axis, such as chromatic dispersion or polarization mode dispersion, cause the transitional areas of the display to close in upon the window from either side.
Active PMD compensation techniques are required in an optical communication system because the PMD of a given fiber varies over time due to the fiber aging, and due to temperature and pressure changes along the fiber. A fiber installed above ground can exhibit fairly rapid fluctuations in PMD due to temperature and mechanical forces (e.g. wind blowing the fiber). A fiber buried underground can be sensitive to loads such as street traffic or construction work. Also, the fiber may not have a perfect circular cross-section, causing varying delays of the polarization components.
In current optical communication systems, PMD changes are typically compensated for by a Polarization Mode Dispersion Compensator (PMDC) which detects the degree of polarization-dependent differential delay suffered by an optical carrier and then adaptively corrects the delay. As polarization characteristics of the fiber change, the PMDC constantly monitors and adjusts the signal in an attempt to minimize the PMD contribution to overall dispersion. A typical PMD compensator splits an incoming optical signal, intensity modulated by a data pulse stream, into two polarizations. The relative timing of the two signal halves is then corrected by introducing a delay into one signal half and then recombining the halves to form a corrected output signal.
Schemes to actively compensate for PMD generally involve detecting the presence of polarization-dependent timing differences and either a) applying delay elements to one or the other polarization to realign the timing of pulses or b) controlling the state-of-polarization (SOP) of the signal upon entry into the fiber, or at intermediate points along the fiber, such that birefringent effects are minimized or canceled out. Existing PMD compensators either include a state-of-polarization (SOP) detector and/or controller in order to feed a consistent polarization orientation into the beam splitter at the front end of the compensator, or rely on the use of one elsewhere in the optical communication system.
A problem of prior art PMD compensators is that they either assume SOP is not changing in the optical path, or utilize a SOP detector and/or controller to stabilize SOP by feeding a consistent polarization orientation into a beam splitter at the front end of the compensator, with the presumption that any residual SOP changes are caused by PMD. These assumptions are incorrect and cause flawed readings and unnecessary corrections of the optical signal to reduce PMD. These assumptions are incorrect because while changes in PMD cause changes in SOP, the converse is not necessarily true; SOP changes are not always caused by PMD. PMD is a separate phenomenon, distinguishable from SOP, where SOP can change dramatically and abruptly without a corresponding change in PMD. Thus, PMD is not being correctly compensated for in current optical transmission systems. More particularly, existing PMD compensator designs assume that SOP through a given optical path does not change appreciably over time, or that SOP can be controlled by a slow response SOP controller such that any residual SOP deviations are presumed attributable to PMD. This assumption is invalid, resulting in poor performance of existing PMD compensators. The present invention, an SOP-independent PMD detector, solves the problems of the prior art by providing a device and method of determining and compensating real-time PMD variations independently of SOP shifts.
The method and device of the present invention analyzes the pulse shape, for example, the eye diagram, of an optical signal on an optical transmission line to determine PMD. The amount of PMD calculated may then be compensated for. The PMD is observable on an eye diagram of an optical pulse stream suffering from PMD as two overlapping pulses, or peaks, with a saddle point in-between. The relative temporal offset, observable as the distance between the two peaks of the eye diagram, is indicative of and a measure of PMD. The SOP is seen on the eye diagram as a function of relative amplitude of the peaks, and advantageously, is not a factor taken into consideration when calculating and compensating for PMD in the present invention.
According to an embodiment of the present invention; disclosed is a state-of-polarization (SOP)-independent device for detecting polarization mode dispersion (PMD) in an optical communication system having an optical transmission line, including a detector coupleable to the optical transmission line, where the detector is capable of analyzing the pulse shape of an optical signal on the optical transmission line to determine PMD of the optical signal. A corrector may be coupled to the detector, where the corrector is capable of compensating the optical signal for the determined PMD. A controller may be coupled to the detector and the corrector. The pulse shape may be an eye diagram which may contain a first peak and a second peak, where the PMD determined by the detector is a function of the distance between the detected first and second peaks. The distance is indicative of a time delay between the detected first and second peaks, which time delay is indicative of first order PMD, also referred to as differential group delay (DGD). The controller may include a memory, and is capable of providing a clock signal to the detector. The device may further include a verifier coupled to the corrector, where the verifier is capable of verifying that the corrector correctly compensates the optical signal for the determined PMD. The corrector is capable of actively compensating the PMD by altering the optical signal as a function of the determined PMD, and the controller is capable of signaling the detector to request analysis of the optical signal.
According to another embodiment of the present invention, disclosed is a state-of-polarization (SOP)-independent device for detecting polarization mode dispersion (PMD) in an optical communication system having an optical transmission line, including a detector coupleable to the optical transmission line, where the detector is capable of analyzing an eye diagram of an optical signal on the optical transmission line to determine PMD of the optical signal. A corrector may be coupled to the detector, where the corrector is capable of compensating the optical signal for the determined PMD. A controller may be coupled to the detector and the corrector, and a verifier may be coupled to the corrector, where the verifier is capable of verifying that the corrector correctly compensates the optical signal for the determined PMD.
According to another embodiment of the present invention, disclosed is a method of detecting polarization mode dispersion (PMD) independent of state-of-polarization (SOP) in an optical communication system communicating an optical signal on an optical transmission line, the optical communication system having at least a detector. The method includes the steps of analyzing the pulse shape which may be an eye diagram of the optical signal using the detector to determine an amount of PMD of the optical signal. The optical signal may be compensated for the determined PMD using a compensator. The optical communication system may include a controller, and the method may further include the step of verifying that the optical signal is compensated correctly for the determined PMD. The method may also include the step of recompensating the optical signal for the determined PMD. Another optional step of this embodiment in which the controller includes a memory is to compare a first determined PMD value to a second determined PMD value, in order to determine whether the second PMD value equals zero or 100% of a pulse width of the optical signal.
The novel method and device for determining PMD independently of SOP provides the advantage of more accurately determining and compensating for PMD in an optical communication system. Improved transmission of optical signals over an optical fiber is provided with the present invention. A further advantage is that the requirement of the use of an SOP controller by either the optical communication system or the PMDC may be eliminated by the present invention, advantageous because of a reduction in the need for an element in the system. A further advantage is that changes in SOP of the optical signal do not affect the amount of PMD determined and compensated for by the system.